The present invention relates to a precision servo valve and more particularly to a hydraulic servo system for treating hydrocephalus.
The treatment of hydrocephalus by means of a servo valve system has been previously described. See, for example, Hakim U.S. Pat. Nos. 3,886,948 and 3,924,635. The servo valve of the present invention was conceived as an improvement over the various servo valves disclosed in those patents. The advantages of the present servo valve, however, will be useful in servo systems generally and particularly in servo systems where very small pressures and volumes are involved since, in almost all servo systems, it is desirable to increase the loop gain and reduce the sensitivity of the system to extraneous variables, advantages which are provided by the construction of the present invention.
With regard to servo valves generally and, in particular, a servo valve intended for use in a hydrocephalus shunt system, it is highly desirable that the valve be very sensitive to the control parameter and relatively insensitive to any other variable, particularly inlet and outlet pressures. While there are certain types of valves that are inherently insensitive to inlet and outlet pressures, e.g. gate valves, ball valves, and the like, they are of relatively complicated construction and typically introduce a substantial element of friction into the system, destroying the accuracy of the servo operation and making it much less sensitive to the control parameter.
For a hydraulic system such as a hydrocephalus shunt servo system, which has to be miniaturized and which must work at relatively low pressures and volumes, the greatly preferred forms of valves are simple check valves in which a valve element works against a valve seat to control flow through the valve, i.e. from the inlet and the outlet. Such valves can be made very reliable, can be highly miniaturized, and further tend to be self-cleaning. Typically, however, such valves are inherently responsive to the pressure differential across the valve, i.e. they are responsive to both inlet pressure and outlet pressure. Indeed, when operated as simple check valves, they are necesarily so. When such valves are then utilized in servo systems, the servo loop must have sufficient gain so that the control variable effects and dominates operation of the opening and closing of the valve in spite of some responsiveness of the valve to inlet and outlet pressures. Thus, if the control signal cannot be made very large, an effort must be made to reduce the operative cross-sectional area of the valve element itself so as to reduce the effect of upstream and downstream pressures. In a system which is already miniaturized, such as a hydrocephalus shunt system, this further complicates the design. The present invention, by reducing sensitivity to undesired variables and mantaining a balance which can be easily controlled by an appropriately applied imbalancing force, allows a small control signal to control relatively large pressures and volumes, even though absolute values are small. In other words, high sensitivity is provided.
Among the several objects of the present invention may be noted the provision of a servo valve which is essentially insensitive to variations in inlet and outlet pressures; the provision of such a valve which is highly reliable; the provision of such a valve in which the valving elements are of relatively simple, check valve type of construction; the provision of such a servo valve which is highly sensitive to the control parameter; the provision of such a valve which is operable at relatively low pressures and relatively low flow volumes and which is therefore suitable for employment in a hydrocephalus shunt system; and the provision of such a valve which is of relatively simple and inexpensive construction. Other objects and features will be in part apparent and in part pointed out hereinafter.